The short answer is yes.
However, bear in mind that since Cloudflare acts as a reverse proxy, there will be two connections -
- Visitors -> Cloudflare
- Cloudflare -> Your server
The trouble is, that the SSL used for point 1, will use Cloudflare’s free Universal SSL, which does not work with Windows XP (sp3) systems.
Which means, even if you install Lets Encrypt on such a setup, your visitors using Win XP will not be able to use your website.
So if your website has a subsequent number of XP users (such as mine, i get around 15% XP users) - Cloudflare’s free plan is not such a good idea.
I’m not sure why Cloudflare won’t use something like Let’s Encrypt instead of Universal SSL - maybe it’s from a business perspective.